Automatic exposure control for controlling photographic shutter

ABSTRACT

A camera shutter is controlled by a capacitive circuit, with a square wave generator of variable frequency and pulse width to control the charging characteristics of the capacitor which is analogized to the reciprocity failure of the camera film.

United States Patent [191 Hansen Jan. 16, 1973 864,320 4/1961 GreatBritain ..250/214 P [76] Inventor: Donald H. Hansen, 1331 N. Forest,

williamsville, 14221 Primary Examiner-James W. Lawrence [22] Filed: Oct7, 197 Assistant Examiner-T. N. Grigsby Attorney-William C. Nealon etal. [2]] Appl. No.: 187,331

[57] ABSTRACT [52] US. Cl. ..250/214 P, 95/10 CT, 250/206 A camerashutter is controlled by a capacitive circuit, [51] Int. Cl ..G0lj 1/00,HOlj 39/12 wi h a square wave generator of variable frequency 53 Fieldof Search 25 214 p 20 95 10 CT and pulse width to control the chargingcharacteristics of the capacitor which is analogized to the reciprocityfailure of the camera film.

10 Claims 3 Drawing Figures "5: LIGHT FLUX l PULSE WIDTH I CONTROL i i i2 l (FORMAT) I0 2 R 3 4 i SQUARE PULSE HEIGHT l sHuTTER I I' wAvECONTROL TRIP SHUTTER GENERATOR (DENSITY) l PHOTOCELL SIGNAL I i PULSE iREPETITION I RATE CONTROL \V I (RECIROCITY) SHUTTER L J "CLOSE" SIGNAL45 I SHUTTER "OPEN" SIGNAL CAMERA SHUTTER I 24 AssocIATEo POWER 5 SUPPLYL.

PATENTEDJAH 16 I915 3.711.721

SHEET 2 [IF 2 CAPACITOR VOLTAGE DUTY CYCLES 0F SQUARE WAVE TIME IN VE NTOR Donald H Hansen Attorney AUTOMATIC EXPOSURE CONTROL FOR CONTROLLINGPI-IOTOGRAPIIIC SHUTTER BACKGROUND OF THE INVENTION The presentinventionrelates in general to photographic shutter and exposure control, andmore particularly to exposure control for long-exposure photographywhich includes a compensation for the reciprocity failure characteristicof the film.

In the prior art, exposure controls which are correlated to variousparameters of the photographic film and to the amount of light fluxincident thereon are known. The most relevant prior art in this regardare exposure controls of the capacitance type. In this type of control,a capacitor is charged at a rate proportional to the illumination towhich the film is subject, for example by exposing a photocell to thesame illumination. The charge or a capacitor is the time integral ofcurrent into the capacitor. Analogously, film exposure is the timeintegral of light flux incident on the film. Therefore, the capacitorintegrates current with respect to time as the photographic filmintegrates incident light flux with respect to time. At a givenpredetermined level of voltage on the capacitor, the camera shutter isclosed. i

In such an arrangement, the speed of the film (ASA) is an analog of thecapacitance. That is, a capacitance having a given charging rate is usedin conjunction with a given ASA film having a given rate at which itbecomes light-saturated.

Prior art exposure controls do not take into account or compensate forthe reciprocity failure characteristic of a film. Reciprocity failure isthat property of a photographic emulsion by which its light-sensitivitydecays during time of exposure. That is to say, the curve of sensitivityvs/exposure time is non-linear. For long exposure times, as are typicalfor example in microphotography, this change of film characteristicbecomes significant.

p SUMMARY OF THE INVENTION It is an object of the present invention toprovide a photographic exposure control which compensates for filmreciprocity failure as well as the other parameters of film speed,format, and density. I

Another object is to provide such an exposure control forphotomicrographic apparatus to enhance the reliability of suchapparatus, particularly where short lived specimens are involved.

Other objects, advantages, and features of this invention will becomeapparent from the following description of one embodiment thereof, takenin connection with the accompanying drawing.

Briefly, the present invention is practiced in one form by aphotographic exposure control operatively connected to a camera shutterand signalling the same to close at a certain voltage level on acapacitor. The charging capacitor simulates the collection of light bythe film and is correlated to the light by a photocell subjected to thesame illumination as is the film. In order to compensate for the filmreciprocity failure, the sensitivity of the photocell is attenuated bycontrolling the current flow therethrough. This is accomplished by asquare wave having a decaying pulse repetition rate which is made tocorrespond to the decaying film speed (reciprocity failure).

DRAWING FIG. 1 is a schematic or block diagram of the system of thepresent invention.

FIG. 2 is a plot of voltage vs. time showing the charging characteristicof the capacitor in response to the input current represented.

FIG. 3 is a curve of capacitor voltage as a function of time.

DESCRIPTION Referring now to FIG. 1, a source of DC voltage is shown at2. It is represented as a battery but could be any suitable DC source,as for example a rectified AC voltage. Source 2 is connected through aswitch 4 to a square wave generator 6. Square wave generator 6 leads, onits output side, to a pulse width control 8, a pulse height control 10,and a pulse repetition rate control 12, all operatively connected to thegenerator 6 to modity its output signal. A photocell 14 isseries-connected in the circuit as shown and is disposed in the camerasystem so as to be subject to the same illumination as the film.Photocell 14 leads through parallel paths to a grounded capacitor 16 anda voltage sensor 18. Voltage sensor 18 is operatively connected to asolenoid 20 which forms a part of a camera shutter 22 to close the samewhen signalled by the voltage sensor 18.

The combination of elements 2, 4, 14, 16, 18, 20, and 22 is consideredtypical of the prior art. In such a prior art arrangement, a constantvoltage is applied to the photocell, and during exposure the photocelloutput is directly related to the illumination on the cell (which is thesame as that on the camera film). At a certain level of voltage, ascharge is collected on the capacitor, a shutter trip signal closes thecamera shutter. In such an arrangement, the exposure time is a functiononly of the time integral of the illuminance on the photocell (andtherefore on the film). Other factors, particularly the reciprocityfailure characteristic of the film, are not compensated for, except fora small amount due to the non-linear charging rate of a capacitor.

In order to provide compensation for the reciprocity failure of thefilm, the sensitivity of the photocell is modulated. That is, its lightresponse is attenuated as a function of time along a curve simulatingthat along which the film response decreases asa function of exposuretime. This control of the photocell response can be accomplished eitherby controlling the photocell illuminance along a suitable curve or bycontrolling the current through the photocell. In the presentlypreferred embodiment of this invention, the photocell is attenuatedelectrically by controlling the current through the cell. This is doneby supplying the photocell with an electrically generated square wave,the pulse repetition rate (frequency), pulse height, and pulse width(duty cycle) of which are all controllable.

The frequency control 12 provides a certain decay of the frequency as afunction of time which rate can be set or adjusted by the operator tocorrespond to the reciprocity failure rate of the film (such reciprocitybeing known). The frequency control 12 thus acts as a function generatorto match its output signal to a nonlinear physical characteristic (i.e.the film reciprocity). This control is effected by a suitable RC controlin the circuit.

The sensitivity of the photocell is further controlled to provide forvarious camera backs (format) and for various film densities that may beused. Pulse width control 8 controls the duty cycle of the square waveand this is correlated to format. Control 8 is an RC control to vary thetime constant of the square wave.

Pulse height control 10 controls the voltage level of the pulses, andthis is correlated to film density or light/dark film characteristic.This control 10 is a variable or selectively changeable resistance inthe circuit.'

Film speed (ASA), or the rate at which the film collects light, is acorrelate of the capacitance or the rate at .which the capacitor 16collects charge. Therefore, various capacitances or capacitors can beselected and switched into the circuit depending on the ASAcharacteristic of the film used. Capacitor 16 may therefore be aplurality of capacitors selectively switchable into the circuit.

' In operation, when switch 4 is closed the camera shutter is opened andthe circuit energized. Light flux incident on the photocell controls thecharging current to the capacitor. When capacitor 16 reaches apredetermined voltage level, the shutter trip signal at 18 signals theshutter trip 20 to close the camera shutter 22. By controlling theoutput of the photocell 14, the charge or voltage collected in thecapacitor 16 can be made to follow a curve with respect to time tosimulate that along which the film collects light with respect to time.Thus, though the film loses sensitivity during time of exposure, thephotocell-controlled capacitor is compensating for this by collectingcharge at a correspondingly decaying rate.

The system described above generally operates with voltages and currentsof signal magnitude only. An associated power circuit 24 preferablywould be included as a practical matter. The details of such a circuitor associated system are not material to this invention however, andneed not be further described.

FIG. 2 shows a plot of voltage as a function of time. This plotrepresents the current output of photocell 14. The pulsesshown are theduty cycles of the square wave and, as is apparent, the frequency of thesquare wave is decaying at a given controlled rate.

FIG. 3 shows a curve of capacitor voltage as a function of time. This isa detail of the charging curve schematically represented in FIG. 1 at16. FIG. 3 is directly beneath FIG.'2 and the time scales areconcurrent.

The photocell output represented in FIG. 2 is the charging potentialwhich is integrated by the capacitor asrepresented in FIG. 3. It will beappreciated that an uninterrupted directcurrent, Le. a duty cycle ofunity, will charge a capacitor along a curve typified by the phantomline. However, the intermittent pulses of the square, wave'jwhich have aduty cycle less than unity, are used in the present inventionto chargethe capacitor ata rate'illustrated by the stepped curve. That is, whenpotential is applied to the capacitor during the pulse of the dutycycle, charge accumulates and during the remaining portion of theperiod, no charge accumulates. Thus, the resultant curve is displaced tothe right as shown and the charging time of the capacitor has beenincreased.

It is apparent from FIGS. 2 and 3 that the charging rate of thecapacitor can be controlled by controlling the output of the photocell.This photocell output can, in turn, be controlled by controlling thedecay of the square wave frequency. Thus, it is possible to electricallysimulate any film reciprocity failure rate. Of course, the frequency ofthe square wave can be controlled so as to decay at any desired rate,linear or nonlinear, the decay rate shown in FIG. 2 merely beingillustrative.

It will be apparent that a photographic exposure control has beenprovided hereby which is able to compensate for the various parametersof film reciprocity failure, film speed, format, and density.

It may occur to others of skill in the art to make modifications of thisinvention which will lie within its concept and scope and not constitutea departure. Accordingly, it is intended that the invention be notlimited by the details of its description, but only by the followingclaims.

What is claimed is:

1. An analog photographic exposure control in which a camera shutter issignalled to close at a determined level of voltage, and including:

a photocell disposed in the camera system and subject to the sameillumination as the film in said camera,

said photocell connected to a capacitor and effective to discharge tosaid capacitor in response to light flux incident on said photocell,

means to vary the charging potential to the capacitor as a function oftime to simulate the reciprocity failure rate of the film.

2. An analog photographic exposure control as defined in claim 1 whereinsaid means includes:

a square wave generator to supply a square wave voltage to saidphotocell,

pulse repetition rate control means to vary the frequency of the squarewave along a desired curve with respect to time to simulate the curvealong which film sensitivity decreases with respect to time duringexposure.

3. An analog photographic exposure control as defined in claim 1 andfurther including:

format control means to modify the charging potential to said capacitorto accommodate for various formats of film which may be used in saidcamera.

4. An analog photographic exposure control as defined in claim 3 inwhich said format control means includes:

a square wave generator to supply a square wave voltage to saidphotocell, and

pulse width control means to control the width of the pulses from saidsquare wave generator.

5. An analog photographic exposure control as defined in claim 1 andfurther including: 7

density control means to modify the charging potential to said capacitorto compensate for small differences in film speed or object contrast.

6. An analog photographic exposure control as defined in claim 5 inwhich said density control means includes:

a square wave generator to supply a square wave voltage to saidphotocell, and

pulse height control means to control the height of the pulses from saidsquare wave generator.

7. An analog photographic exposure control in which a camera shutter issignalled to close at a determined level of voltage, and including:

6 a photocell disposed in the camera system and sub- 8. An analogphotographic exposure control as ject to the same illumination as thefilm in said defined in claim 7 wherein said means to apply the camera,charging potential to said capacitor includes: said photocell connectedto acapacitor and effective a Square wave generator to supply a squarewave to discharge to said capacitor in response to light 5 voltage tosaid photocell, and flux incident on said photocell, said control meansto vary the frequency of the pulmeans to apply the harging tenti l t idises includes pulse repetition rate control means to tor at a dut cl l hunity, vary the frequency of the square wave with respect control meansto vary the frequency of the pulses to to timesimulate the reciprocityfailure rate of the film dur- An analog P p exposure control as iexposure, defined in claim 8, wherein the frequency of the squarecontrol means to vary the duration of said pulses to Wave decays at anon-near accomodate various formats of film, and anfilog P g p exposurecontrol as control means to vary the pulse voltage to compen defined inclaim 8 wherein the frequency of the square sate for small differencesin film speed or object wave decays atalmear ratecontrast.

1. An analog photographic exposure control in which a camera shutter issignalled to close at a determined level of voltage, and including: aphotocell disposed in the camera system and subject to the sameillumination as the film in said camera, said photocell connected to acapacitor and effective to discharge to said capacitor in response tolight flux incident on said photocell, means to vary the chargingpotential to the capacitor as a function of time to simulate thereciprocity failure rate of the film.
 2. An analog photographic exposurecontrol as defined in claim 1 wherein said means includes: a square wavegenerator to supply a square wave voltage to said photocell, pulserepetition rate control means to vary the frequency of the square wavealong a desired curve with respect to time to simulate the curve alongwhich film sensitivity decreases with respect to time during exposure.3. An analog photographic exposure control as defined in claim 1 andfurther including: format control means to modify the charging potentialto said capacitor to accommodate for various formats of film which maybe used in said camera.
 4. An analog photographic exposure control asdefined in claim 3 in which said format control means includes: a squarewave generator to supply a square wave voltage to said photocell, andpulse width control means to control the width of the pulses from saidsquare wave generator.
 5. An analog photographic exposure control asdefined in claim 1 and further including: density control means tomodify the charging potential to said capacitor to compensate for smalldifferences in film speed or object contrast.
 6. An analog photographicexposure control as defined in claim 5 in which said density controlmeans includes: a square wave generator to supply a square wave voltageto said photocell, and pulse height control means to control the heightof the pulses from said square wave generator.
 7. An analog photographicexposure control in which a camera shutter is signalled to close at adetermined level of voltage, and including: a photocell disposed in thecamera system and subject to the same illumination as the film in saidcamera, said photocell connected to a capacitor and effective todischarge to said capacitor in response to light flux incident on saidphotocell, means to apply the charging potential to said capacitor at aduty cycle less than unity, control means to vary the frequency of thepulses to simulate the reciprocity failure rate of the film duringexposure, control means to vary the duration of said pulses toaccomodate various formats of film, and control means to vary the pulsevoltage to compensate for small differences in film speed or objectcontrast.
 8. An analog photographic exposure control as defined in claim7 wherein said means to apply the charging potential to said capacitorincludes: a square wave generator to supply a square wave voltage tosaid photocell, and said control means to vary the frequency of thepulses includes pulse repetition rate control means to vary thefrequency of the square wave with respect to time.
 9. An analogphotographic exposure control as defined in claim 8, wherein thefrequency of the square wave decays at a non-linear rate.
 10. An analogphotographic exposure control as defined in claim 8, wherein thefrequency of the square wave decays at a linear rate.